Integrated electronic circuits fabricated on a semiconductor chip typically include a number of hard-coded bits for various purposes. For example, a revision identifier on a semiconductor chip includes a number of hard-coded bits to indicate which stepping of the mask is used to fabricate the chip. A new stepping of the mask is created every time the layout of the semiconductor chip is changed, and a distinct revision identifier, also known as a stepping ID, is assigned to each stepping. Typical examples of revision identifier include “A0,” “A1,” “B1,” or “B2,” etc.
The layout of an integrated circuit is changed when a functional change is made in the integrated circuit. Changing the layout requires creating a new stepping. Consequently, the revision identifier has to be changed as well when a functional change is made in the integrated circuit.
Currently, there are two methods to implement a hard-coded bit value in a semiconductor chip. The first method implements the hard-coded bit value as a part of the register transistor level (“RTL”) code. Since the hard-coded bit value is generated by the RTL code, the bit is not associated with one and only one metal layer. Therefore, multiple metal layers are modified to change the bit.
The second method to implement a hard-coded bit value in a semiconductor chip is to fabricate a custom-built metal structure on the semiconductor chip to couple Vss or Vcc to the hard-coded bit. FIG. 1A shows a metal structure 130 on a semiconductor chip (not shown) with a hard-coded bit 110 at one end of the metal structure 130. The hard-coded bit 110 is coupled to Vss 140 via a connection 120 in between the metal structure 130 and Vss 140, and therefore, the hard-coded bit 110 is set to logic 0. Changing the metal layers of the semiconductor chip changes the metal structure to cause a change in the hard-coded bit. However, once a connection is created in a metal layer, subsequent changes of the metal structure requires cutting off the connection regardless of whether the functional change is in the same metal layer as the connection is in. In other words, two or more metal layers have to be changed in order to change the hard-coded bit.
Suppose the functional change in the new stepping requires changing only the metal layer 160. The new stepping also needs a new revision identifier. Therefore, the hard-coded bit 110 on the metal structure 130 has to be changed. Referring to FIG. 1B, the metal layer 150 is changed to remove the connection 120 in order to cut off the metal structure 130 from Vss 140. In addition, a connection 170 is fabricated in metal layer 160 to couple the metal structure 130 to the Vcc 180 to change the hard-coded bit from logic 0 to logic 1. It is necessary to remove the connection 120 in the metal layer 150 to allow a change in the hard-coded bit 110 on the metal structure 130 regardless of which metal layer is changed to implement the functional change in the new stepping. As a result, two metal layers are changed even though the functional change requires changing only one metal layer.
As explained above, the change of the revision identifier associated with a new stepping is not necessarily in the same metal layer as the functional change in the integrated circuit using the current methods. Frequently, changing the revision identifier requires changing more metal layers than the functional change requires. Changing a metal layer typically costs $10,000. As a result, the more metal layers are changed in a stepping, the more expensive the stepping is.